Drilling fluids used in the drilling of subterranean oil and gas wells along with other drilling fluid applications and drilling procedures are known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply “muds”. The drilling fluid is expected to carry cuttings up from beneath the bit, transport them up the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and also often to form a thin, low permeability filter cake which temporarily seals pores, other openings and formations penetrated by the bit. It is desirable to minimize the loss into the formation of the relatively expensive drilling fluid. Drilling fluids must also be formulated to stabilize the wellbore wall to keep it from swelling, for instance in the case of drilling through shale. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs, thus its electrical properties are important. Drilling fluids should also not unnecessarily aggravate the tendency of drill bits, tubulars and other downhole equipment to corrode, and, in a particular non-limiting embodiment, should help prevent such corrosion. It will be appreciated that within the scope of the claimed invention herein, the term “drilling fluid” also encompasses “drill-in fluids”, “completion fluids”, “workover fluids” and remediation fluids.
It is apparent to those selecting or using a drilling fluid for oil and/or gas exploration, and field development that an essential component of a selected fluid is that it be properly balanced to achieve the necessary characteristics for the specific end application. Because drilling fluids are called upon to perform a number of tasks simultaneously, this desirable balance is not always easy to achieve. It is also important for the properties of the drilling fluid to be stable, for instance that the rheological properties (viscosity, etc.) are stable throughout the pressure and temperature ranges that the fluid experiences, possibly including high temperature, high pressure conditions which are abbreviated HTHP.
Drilling fluids are typically classified according to their base fluid. In water-based muds, solid particles are suspended in a continuous phase consisting of water or brine. Oil can be emulsified in the water which is the continuous phase. Brine-based drilling fluids, of course are water-based mud (WBM) in which the aqueous component is brine. Oil-based muds (OBM) are the opposite or inverse of water-based muds. In oil-based muds, solid particles are suspended in a continuous phase consisting of oil. Water or brine can be emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds can be either all-oil based or water-in-oil macroemulsions, which are also called invert emulsions. In oil-based mud, the oil may consist of any oil or water-immiscible fluid that may include, but is not limited to, diesel, mineral oil, esters, refinery cuts and blends, or alpha-olefins. OBMs as defined herein also include synthetic-based fluids or muds (SBMs) which are synthetically produced rather than refined from naturally-occurring materials. SBMs often include, but are not necessarily limited to, olefin oligomers of ethylene, esters made from vegetable fatty acids and alcohols, ethers and polyethers made from alcohols and polyalcohols, paraffinic, or aromatic, hydrocarbons alkyl benzenes, terpenes and other natural products and mixtures of these types.
Formation damage involves undesirable alteration of the initial characteristics of a producing formation, typically by exposure to drilling fluids, completion fluids or in the production phase of the well. The water or solid particles in the fluids used in drilling and completion operations tend to decrease the pore volume and effective permeability of the producible formation in the near-wellbore region. There may be at least three possible mechanisms at work. First, solid particles from the fluid may physically plug or bridge across flowpaths in the porous formation. Second, when water contacts certain clay minerals in the formation, the clays typically swell, thus increasing in volume and in turn decreasing the pore volume. Third, chemical reactions between the fluid and the formation rock and fluids may precipitate solids or semisolids that plug pore spaces. Phase transitions due to changes in pressure or temperature of fluid composition during the wellbore construction and production may lead to undesirable precipitation or formation of asphaltenes, wax, scales, etc.
Reduced hydrocarbon production can result from reservoir damage when a drilling mud deeply invades the subterranean reservoir. It will also be understood that the drilling fluid, e.g. oil-based mud, is deposited and concentrated at the borehole face and partially inside the formation. Many operators are interested in improving formation clean up and removing the cake or plugging material and/or improving formation damage after drilling into reservoirs with oil-based muds.
It is also important when drilling subterranean formations to keep the wellbore stable, so that the walls of the borehole do not cave into the hole, and that the stability of the walls is maintained. Other issues involve improving the electrical resistivity or otherwise modifying the electrical conductivity of the drilling fluid. In some cases, it is desirable to diminish the drilling fluid resistivity, that is, improve the inverse property or the electrical conductivity of the fluid.
It would be desirable if drilling fluid compositions and methods could be devised to avoid damage to the near-wellbore area of the formation, as well as assess the location and existence of damage and aid and improve the ability to clean up damage and difficulties caused to the wellbore, the formation, equipment in the wellbore (for instance, stuck pipe), and to remove and/or resolve problems more completely and easily, without causing additional damage to the formation, wellbore and/or equipment.